Good Morning,

SLA Definitions can be found under the following menu

1) Service Level Management > SLA > SLA Definitions

2) Search and Open the SLA Definitions you want to update

3) In the field 'Schedule' is where you set your holidays

4) Open Reference field Schedule

5) Open the Related List at the bottom of the page for the tab: Schedule Entries

6) Add Holidays in the Schedule Entires tab and set Type to Exclude

If the tab Schedule Entries isn't there, configure the form and add it.

I hope that helps

Kind Regards

Ashley